Cement kilns have received favorable review from both federal and state environmental regulatory agencies for disposal of both liquid and solid combustible waste. Cement kilns provide a combination of high operating temperatures and long residence times, both favorable conditions for complete combustion of organic components of waste and chemical combination of inorganic components with the reactive in-process mineral components. In spite of the availability of a wide variety of combustible waste solids as a source of inexpensive energy for the mineral processing industry, perceived engineering problems, including concerns about waste handling, end product quality and emission control, have until recently deterred kiln operators from using solid combustible waste as supplemental fuels for operating cement kilns.
For many years, regulation compliant use and disposal of wastes in operating kilns has been limited to combustible liquid or "pumpable" hazardous waste. Liquid waste materials are easily blended with each other and with conventional fuels to provide homogenous liquids that can be burned in the gaseous phase at the firing end of the kiln with little or no modification of kiln burner configuration. Solid waste, however, can occur in a multiplicity of forms, from hard crystalline solids to viscous, sticky sludges. They are not easily blended, and they present significant engineering challenges for their safe handling and delivery into rotary kilns. One solution to such problems is described in U.S. Pat. 4,850,290, issued July 27, 1989, incorporated herein by reference, which patent describes the delivery of containerized waste to both pre-heater type and conventional long wet or dry kilns at a point in the process where the kiln gas temperature is such that volatilized components are consumed with high destruction and removal efficiency. That patent also describes a device for delivering containerized waste through the wall of a kiln cylinder during kiln operation. The apparatus comprises a port, preferably with a mechanical closure, in the kiln cylinder wall; the port is aligned with a drop tube inside the kiln cylinder. The drop tube prevents hot mineral material in the kiln from escaping through the port or contacting the closure. The apparatus is utilized to deliver containerized waste into the kiln at predetermined times during kiln cylinder rotation.
Although the use of containerized solid waste in cement kilns has proven to be an economical, environmentally sound method for disposal and energy recovery from solid waste, particularly solid hazardous waste, it has been found that there are many forms or types of solid waste which need not be containerized to ensure their effective energy-saving use in operating cement kilns. Thus rubber tires, for example, have been used as an alternate source of fuel for cement kilns since 1978 in Europe and more recently in the United States. See, for example, U.S. Pat. Nos. 4,551,051 and 4,256,503. They have been used most effectively by their delivery, as whole tires or in chipped form, into the hot transition zone between the pre-heater and the rotating kiln cylinder of pre-heater kilns.
The present invention is directed to an apparatus useful for delivering combustible solids, particularly used tires, without containerization, through a kiln cylinder wall and into a hot zone of an operating kiln. One problem presented by delivery of non-containerized combustible waste solids to operating cement kilns, which is not such a problem with containerized waste, derives from the direct contact of the combustible waste with the hot port closure immediately prior to delivery of the combustible solids into the kiln. During kiln operation the port closure can reach temperatures capable of melting or even effecting premature decomposition or combustion of the waste solids in contact with the closure. That problem is addressed in the improved charging device of the present invention by configuring the closure to have inner and outer portions defining a passageway for directed closure cooling air flow into the kiln.
Thus, according to one embodiment of the present invention, an apparatus is provided for charging combustible solids through a port in the wall of a rotating kiln into the heated zone of the kiln. The apparatus comprises means for positioning combustible solids for passage through the port, a closure for the port, and means for moving the closure between a port-closed position and a port-opened position. The closure comprises an outer portion and an inner portion which portions cooperate, at least when the closure is in the port-closed position, to define a passage in air-flow communication with both the heated zone of the kiln and ambient air. The flow of air through the passage cools at least part of the surface of the outer portion of the closure to minimize premature volatilization or decomposition of combustible solids positioned against the closure for passage through the port. Preferably, the inner and outer portions are independently movable between positions corresponding to port-opened and port-closed positions.
A drop tube extending from the port into the heated zone of the kiln prevents hot mineral material in the kiln from escaping through the port or contacting the closure. Preferably, the drop tube and port are sized to receive whole tires. In another preferred embodiment, the present apparatus includes a stationary staging assembly for supporting combustible solids before delivery to the kiln and a transfer assembly mounted on the kiln wall for receiving solids from the staging assembly and aligning them with the port in the kiln wall.